1. Field of the Invention
The present invention relates to a camera provided with a movable mirror.
2. Description of the Related Art
It is known that a conventional mirror driving mechanism for a single-reflex camera makes use of a method of rotating a movable mirror by utilizing the unidirectional operation of a motor, such as that disclosed in U.S. Pat. No. 4,864,336.
In this disclosed method, rotation of the movable mirror from an exposure-retracted position (hereinafter referred to as a "mirror-up position") to a finder-viewing position (hereinafter referred to as a "mirror-down position") is effected by means of a spring force, while rotation of the movable mirror from the mirror-down position to the mirror-up position is effected by means of the driving force of the motor.
However, it has been found that a camera having the above-described mirror driving mechanism involves a number of disadvantages.
One disadvantage is that, as the level of source voltage decreases with the consumption of a battery used as a power source, the number of revolutions of the motor which is driven for moving up the movable mirror is reduced and the time period required to move the movable mirror to the mirror-up position lengthens. The result is an increase in a release time lag.
Another disadvantage pertains to a moving-subject predictive autofocus mode which has recently been developed as one kind of autofocus mode, such as that disclosed in U.S. patent application Ser. No. 259,783 filed on Oct. 19, 1988. In such an autofocus mode, although a reduction in and the stabilization of the release time lag are important considerations, the use of the conventional mirror driving mechanism cannot necessarily meet these considerations.
Another disadvantage resides in the detection of the timing when the movable mirror reaches the mirror-down position. In the conventional mechanism, when a shutter charging operation is completed and energization of a shutter charging motor is stopped, a distance-metering operation is started. As a result, it is difficult to improve the frame speed at which continuous shooting is carried out while an autofocus operation is being performed. To cope with this disadvantage, the art of detecting through an exclusive detecting switch the completion of the mirror-down operation of the movable mirror through an exclusive detecting switch is known in Japanese Laid-Open Patent Application No. Sho 57-73725.
Another disadvantage is that, since a driving power for moving the movable mirror downward is obtained from the spring force, it is difficult to suppress the bounds of the movable mirror which occur when it reaches the mirror-down position.
More specifically, if the spring force is strengthened, a retaining force for retaining the movable mirror in the mirror-down position must be strong. However, the speed at which the movable mirror strikes a mirror stopper increases, with the result that bounds cannot be suppressed and impact noise increases.
If the spring force is weakened, the speed at which the movable mirror strikes the mirror stopper can be reduced, but the retaining force for retaining the movable mirror in the mirror-down position weakens. As a result, no strong force for suppressing bounds can be obtained and a long time period will elapse until the bounds disappear.